1. Technical Field
The present invention relates to a circuit board, and more particularly to a multilayer circuit board.
2. Description of Related Art
Printed circuit board (PCB) is widely used in electronic devices. In early days, PCB is a single substrate laminated with a circuit on a side or opposite sides thereof. Now, multilayer PCB is popular in the present market, which has several substrates in stack, and the circuit is embedded therein. FIG. 1 shows a conventional multilayer PCB 300, including several substrates 310 in stack, interconnectors 320 (usually are plated through holes, PTH) to connect circuits 340 of each substrate 310, and pads 330. The pads 330 are provided on an outer surface of the PCB 300, and electrically connected to the interconnectors 320. In order to test the connection of the circuits 340, probes 350 are provided to touch the pads 330. A test signal is transmitted to the circuits 340 from the probes 350 to test the electrical connection.
Typically, the test signal is transmitted in a section of the interconnectors 320, so called functional sections, between the pads 350 and a point where the circuits 340 connect to the interconnectors 320. The other sections of the interconnectors 320, so called null sections 320a, give no function on transmission of the test signal. However, there is a resonance occurred in the null sections 320a while high-frequency signal is transmitted through the null sections 320a, and that will interfere with the signal on the functional sections and result an error in the test.